The present invention relates to an improved billet and process for producing a tubular body by forced plastic deformation and more particularly relates to improvements in the process for producing a wide variety of tubular bodies such as heat pipes by a novel combination of the use of a billet of a core and sheath construction with use of conventional forced plastic deformation such as hydrostatic extrusion, swagging and drawing under process conditions similar to those employed for processing solid rod bodies.
In one known process for producing a tubular body by forced plastic deformation, applied to a billet of the core and sheath construction, a billet comprising at least one axially elongated salt core and an insoluble sheath pipe wholly embracing the core is prepared, the billet so prepared is then subjected to a forced plastic deformation such as hydrostatic extrusion to reduce the diameter of the billet, and, finally the salt core is removed through solution (e.g., by blowing steam through the core) in order to obtain the tubular body.
In cases where the foregoing process is employed in the production of tubular bodies in which the extent of the diametric reduction (i.e., the compaction ratio) is very large, or in cases where tubular bodies having numerous axially elongated radial fins are to be reproduced, several technical problems are encountered. Particularly, as a result of the crystallinity of the salt used as the core material, it is difficult to obtain high precision results. That is, it is quite difficult to achieve a high degree of similarity between the shape of the original material sheath pipe and the final tubular body. Further, when the foregoing known billet is used, the material used for the core (salt) is removed through solution at the final stage of the process. This removal of the core inevitably causes two technical problems: large consumption of the core material and pollution of the environment as a result of the disposal of the salt core.
A second known process for producing a tubular body by forced plastic deformation applied to a billet of the core and sheath construction is disclosed in U.S. Pat. No. 1,390,746 issued to Armstrong. The Armstrong patent discloses a billet having a relatively thick outer sheath which encapsulates a complex core including a metallic central core and a refractory intermediate core disposed between the outer sheath and the inner core. In accordance with the process disclosed in Armstrong, a billet of the foregoing construction is subjected to classic deformation to produce the diameter thereof. Thereafter, the inner metallic core is mechanically removed and the intermediate refractory core is removed by reaming. The billet and process of the Armstrong patent have two primary drawbacks. Initially, since the intermediate refractory core is not easily soluble, it must be removed by reaming. Additionally, due to the mechanical properties of the refractory material, it has been found that satisfactory results cannot be obtained when the ratio of the sheath thickness T to the outer diameter of the pipe is equal to or less than 0.11. Tests establishing these results are reviewed in some detail below.